The present application is based on Japanese Patent Application No. 2001-106561, which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to an optical film in which axial displacement of the laminate hardly occurs in spite of the change of a view point so that the optical film can be preferably used for forming a liquid-crystal display device good in display quality, a circularly polarizer or an anti-reflection plate.
2. Description of the Related Art
If a phase retarder to be disposed between a polarizer and a liquid-crystal cell in order to improve display quality of a liquid-crystal display device, or a quarter-wave plate used for forming a circularly polarizer or an anti-reflection plate is formed from one birefringent film, birefingence is dispersed in accordance with the wavelength of light on the basis of dispersion peculiar to the material of the retarder or quarter-wave plate. As a result, there is a tendency that the birefringence increases as the wavelength decreases. For this reason, the retardation of the plate varies in accordance with the wavelength of light, so that the state of polarization cannot change evenly. Under such circumstances, there has been a proposal for an optical film having a laminate of two birefringent films different in birefringence dispersion depending on the wavelength of light so that respective slow axes of the two birefringent films cross each other perpendicularly (Unexamined Japanese Patent Publication No. Hei. 5-27118 and Unexamined Japanese Patent Publication No. Hei. 10-239518).
The proposal is aimed at controlling birefringence dispersion depending on the wavelength of light by the lamination of the birefringent films so that the birefringene decreases as the wavelength decreases. That is, the proposal is provided for obtaining a uniform compensating effect in achieving a uniform change of the state of polarization in a wide wavelength range. Although the perpendicularly crossing relation can be kept so as to fulfill the required effect when observed on an optical axis, the perpendicularly crossing relation is corrupted because of the change of the apparent axial angle when observed from an oblique direction at an azimuth displaced from the optical axis. Hence, there is a problem that the state of polarization varies without fulfillment of the required effect. Even in the case where Nz values of the birefringent films are controlled to compensate for axial displacement from the polarizer as disclosed in the aforementioned JP 5-27118 this measure is not effective in compensating for axial displacement of the birefringent film laminate itself.
An object of the invention is to develop an optical film in which the axial relation between slow axes crossing each other perpendicularly can be kept good in spite of the change of a view point so that the optical film can be used for forming a liquid-crystal display device good in display quality, a quarter-wave plate, etc.
According to the invention, there is provided an optical film having a laminate of three birefringent films A, B and C, the birefringent film A exhibiting refractive index dispersion in accordance with the wavelength of light and exhibiting Nz of from 0.4 to 0.6, the birefringent film B exhibiting larger refractive index dispersion in accordance with the wavelength of light than that of the birefringent film A and exhibiting smaller Re that that of the birefringent film A and NZ of from 0.4 to 0.6, the birefringent film C exhibiting Re of from 200 to 350 nm and exhibiting Nz of from 0.6 (not inclusively) to 0.9, the birefringent films A and B having slow axes crossing each other perpendicularly when Re=(nxxe2x88x92ny)d and Nz=(nxxe2x88x92nz)/(nxxe2x88x92ny) in which nz is a refractive index of each birefringent film in a direction of a Z axis expressing a direction of the thickness of the birefringent film, nx is a refractive index of the birefringent film in a direction of an X axis expressing a direction of the maximum refractive index of the birefringent film in a plane perpendicular to the Z axis, ny is a refractive index of the birefringent film in a direction of a Y axis expressing a direction perpendicular both to the X axis and to the Z axis, and d is the thickness of the birefringent film. There is also provided a liquid-crystal display device having a liquid-crystal cell, and at least one optical film defined above and disposed on at least one surface of the liquid-crystal cell.
According to the invention, there is further provided a polarizer having a laminate of an optical film defined above and an absorptive type polarizing film, the absorptive type polarizing film being laminated on the birefringent film C side of the optical film so that an absorption axis of the absorptive type polarizing film is parallel to a slow axis of the birefringent film C in the condition that the birefringent film C is located in an outer side of the optical film. There is further provided a liquid-crystal display device having a liquid-crystal cell, and at least one polarizer defined above and disposed on at least one surface of the liquid-crystal cell so that the absorptive type polarizing film of the polarizer is located in an outer side. There is further provided a display device having a polarizer defined above, wherein: an in-plane retardation of the laminate of the birefringent films A and B is in a range of from 80 to 400 nm; and the polarizer is disposed on an outermost surface so that the absorptive type polarizing film of the polarizer is located in an outer side.
According to the invention, in addition to the characteristic that the retardation due to birefringence hardly changes on an optical axis can be retained by the combination of the birefringent films A and B and the arrangement relation between the birefringent films A and B, the perpendicularly crossing relation between optical axes can be retained accurately even in the case where the view point is changed within 360 degrees. It is accordingly possible to obtain an optical film which can fulfill a homogeneous compensating effect even in the case where the optical film is observed at any azimuth and in which the birefringent film C can compensate for axial displacement from the polarizing film in an obliquely viewing direction to thereby prevent the optical axis of the optical film from changing. As a result, the optical film can be used for forming a liquid-crystal display device good in display quality. Moreover, such an optical film functioning as a quarter-wave plate can be used in combination with an absorptive type polarizing film for obtaining a circularly polarizer or an anti-reflection plate in which the uniform change of the state of polarization in a wide wavelength range can be achieved to obtain a uniform compensating effect even in the case where observation is made at any azimuth. The circularly polarizer or the anti-reflection plate can be used for obtaining various kinds of display devices good in display quality.
Features and advantages of the invention will be evident from the following detailed description of the preferred embodiments described in conjunction with the attached drawings.